Alga- and Lichen-Stabilized Surface Crusts as Soil Nitrogen Sources

Shields, Lora; Mitchell, Charles; Drouet, Francus

A series of observations establishes the incidence of a total of 14, algal species in alga- and lichen-stabilized soil crusts from several contrastlng semi-desert substrates. These observations provide the basis for evaluating such surface crusts as sources of amino, nitrogen and total nitrite and nitrate. 1) On flats throughout the gypsum sand deposit the surface crust is stabilized mainly by Palmogloea-protuberans, associated occasionally with Plectonema nostocorum and rarely with Nostoc. 2) Amino nitrogen averaged 256 p.p.m. for alga-stabilized gypsum crust and 10 p.p.m, for the 6-in. level in June. Total nitrite and nitrate averaged 5.6 p,p.m, for algal crust samples in June, 0.45 p.p.m. 6 in. below the algal crust, and 2.1 p.p.m. for samples from 3- to 6-in. levels in the vicinity of roots. 3) More than half the gypsum samples exhibited nitrifying activity, but no evidence could be found for bacterial nitrogen fixation. 4) In 109 surface crusts from lava soil, the more prominent algal species, in order of decreasing incidence, were Microcoleus vaginatus, Porphyrosiphon fuscus, Schizothrix californica, Scytonema ocellatum, S. hofmannii, Schizothrix giuseppei, Microcoleus paludosus, and Nostoc spp. The same algae appeared' in the 50 algal crusts as in the 59 which were lichen-stabilized. The lichen species found on lava soils, in approximate order of diminishing frequency of occurrence, were Dermatocarpon squamellum, Heppia despreauxii, Collema coccophorum, Lecidea decipiens F. dealbata, Collema nigrescens, Lecidea rubiformis, Heppia guepinii, and Solorina saccata var. spongiosa. 5)The amino nitrogen of the lichen crusts from lava soil averaged 22.2 per cent higher (1985 p.p.m.) and the total nitrite and nitrate 21.5 per cent lower (6.2 p.p.m.) than for the alga-stabilized stratum. 6) Culture in 2 selective media demonstrated the presence of nitrifying bacteria in most lava soil samples and Azotobacter in somewhat less than half. The amino nitrogen averaged 866 p.p.m, for lava surface layers lacking algal or lichen growth, however, compared to 1545 and 1985 p.p.m., respectively, for strata supporting algae and/or lichens. 7) The algal species characteristic of surface crusts from the Tularosa Basin floor immediately surrounding the gypsum sand belong to the blue-green genera Microcoleus, Schizothrix and Scytonema. Amino nitrogen averaged 1031 p.p.m. for algal crusts, 1553 p.p.m. for lichen crusts, and 199 p.p.m. at the 6-in. level below lichens. 8) In the mesquite and creosote bush zones, in the absence of algal or lichen growth, amino nitrogen apparently from organic debris averaged 225 p.p.m., and total nitrite and nitrate, 2.1 p.p.m. for surface strata. Amino nitrogen averaged 270 p.p.m. and total nitrite and nitrate 2.5 p.p.m. at the 6-in. level. 9) Alga- and lichen-stabilized surface crusts, through death and decomposition of component cells, particularly during periods of desiccation, release amino and other nitrogen compounds. These are mineralized when moisture becomes available and are distributed with soil water. The surface growth of algae and lichens thus represents a continually renewable supply of soil nitrogen.

Alga- and Lichen-Stabilized Surface Crusts as Soil Nitrogen Sources

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